Beam shaping refers to the transformation of the intensity profile of an initial input beam into a distinct profile a distance away from the beam-shaping devices. Often, beam shaping is desired in such a form that it deviates considerably from the natural shaping provided by free propagation alone (i.e., diffraction). Consequently, it often becomes necessary to employ beam-shaping devices to modify the nature of the propagating beam and thereby provide the desired shaping function.
A simple form of beam shaping and homogenization is provided by Gaussian diffusers, which include a surface with random height variations. Ground glass and some types of chemically etched glass surfaces are used to provide such random height variations. Gaussian diffusers uniformly spread an input illumination beam over a limited angle range with Gaussian intensity profile. Such beam shapers are inexpensive and easy to fabricate but provide very limited beam-shaping capabilities.
Another type of diffusion-based beam shaper having homogenization capabilities can be fabricated by holographic exposure of laser speckle patterns. These so-called “holographic diffusers” provide some advantages over Gaussian diffusers by providing more flexibility in beam shaping, such as by spreading light with distinct angular divergence along two directions. Exact divergence can also be better controlled. However, the typical intensity scatter profile for holographic diffusers is also Gaussian. Other intensity profiles could in principle be obtained; but the holographic method of fabrication assumes a device with the desired intensity profile already exists, which limits the usefulness of the method. Furthermore, in reconstruction, a zero order (straight-through) beam would also be present in addition to the desired pattern. These drawbacks limit the usefulness of holographic components to anything other than a Gaussian spread of light.
Another approach to achieving beam shaping and homogenization is based on diffractive elements, which use interference and diffraction effects to shape an input beam into a variety of patterns. Problems with diffractive elements arise when large divergence angles are required, since diffractive elements achieve light spreading by reducing surface feature sizes (small features lead to large scattering angles). As divergence angles increase, it becomes harder to fabricate diffractive elements, which are typically limited to angles below ±20 degrees. Diffractive elements are also best suited for monochromatic operation and are generally designed to operate at a specific wavelength. At other wavelengths, a strong undiffracted zero-order beam component appears. Diffractive elements can be designed to operate at discrete wavelength values; but for broadband operations, such devices offer poor performance, with the zero order being the main source of degradation.
Beam flattening, such as the conversion of an incident Gaussian beam into a beam that presents flat intensity over some angular span, can also be undertaken by diffractive elements, which suffer the disadvantages mentioned previously. Aspheric lenses have also been used for beam flattening; but aspheric lenses present difficulties relating to fabrication, alignment, limited depth of field, and sensitivity to input beam variations.
Regular microlens arrays have been previously used for near-field homogenization, but these arrays produce strong diffraction patterns away from the array as well as image artifacts such as moire effects in screen applications. Regular microlens arrays have also been used for illumination purposes but provide limited spatial shaping (polygonal energy distribution) and limited intensity control (spherical or aspheric lens profiles on a regular array) away from the array.
Some other beam-shaping transformations require the exclusion of light from some portions of the scattered pattern (i.e., “holes” in the scatter pattern). Except for diffractive elements, prior methods have been unable to provide beam-shaping capabilities that include such multiply-connected scatter patterns.